1. Field of the Invention
The present invention concerns an opto-magnetic recording medium such as an opto-magnetic disk and, more in particular, it relates to an opto-magnetic recording medium having a perpendicularly magnetized film of a dual layer structure comprising a first layer (reading layer) and a second layer (writing layer) each composed of rare earth-transition metal alloy film.
2. Description of the Related Art
The opto-magnetic recording medium using a thin amorphous alloy film comprising a rare earth metal (RE) and a transition metal (TM) has no sufficient reading characteristics and various improvements have been studied therefor.
As an example, there has been proposed a curie point writing type opto-magnetic recording medium mainly constituted with an exchange-coupled perpendicularly magnetized film of a dual layer structure comprising a writing layer having a low curie point and a large coersive force and a reading layer having a higher curie point and a smaller coersive force as compared with those of the writing layer (Japanese Patent Laid Open Sho 57-78652, "III amorphous multi-layered film", p 294-305 in, Collected Technology for Optical Memory-Opto-Magnetic memory, published from Science Forum Co. Ltd, Oct. 31, 1983).
In the conventional exchange-coupled dual layer film as described above, TbFe, DyFe, TbFeCo, DyFeCo are used for the writing layer and GdFe, GdFeCo, TbFeCo, etc. are used for the reading layer. For improving the C/N ratio which is a ratio between a carrier level and a noise level in an opto-magnetic recording medium thereby improving the reading characteristics, it is necessary to increase the carrier level and reduce the noise level.
Among them, the carrier level may be increased by making the Kerr rotational angle greater in the reading layer, while it is important to increase the exchange-coupling force between the two layers for reducing the noise level.
By the way, for improving the carrier level, it is necessary to use a medium having a great Kerr rotation angle for the reading layer, but a medium of a greater Kerr rotation angle generally has increased saturation magnetization. However, as the saturation magnetization is increased, the recording characteristics of the reading layer become predominant during recording, tending to cause writing noises. In this case, although the carrier level is increased, the noise level is also increased by so much, thereby giving no contribution to the improvement of the C/N ratio.
In view of the above, it is an object of the present invention to improve the C/N ratio as an opto-magnetic recording medium by developing a medium for the reading layer having a great Kerr rotation angle while suppressing the increase of the saturation magnetization as less as possible and controlling the sublattice magnetization for each of the rare earth and the transition metal in each of the writing layer and the reading layer, thereby suppressing the occurrence of writing noises.
The dual layer medium includes, as shown in FIGS. 2(a) and (b), a type in which the magnetization Ms1 and Ms2 in the reading layer RL and the writing layer WL, respectively, are in parallel with each other (hereinafter referred to a P type) and, as shown in FIGS. 3(a) and (b), a type in which the magnetization Ms1 and Ms2 in the reading layer RL and the writing layer WL, respectively, are opposite to each other (hereinafter referred to as A type).
For the dual layer film medium of the P type, as shown in FIGS. 2(a), (b), sublattice magnetization Ms.sub.RE for the rare earth RE is predominant in both of the reading layer RL and the writing layer WL (hereinafter the state in which the auxiliary magnetization Ms.sub.RE for the rare earth RE is predominant is referred to as RE rich), or sublattice magnetization Ms.sub.TM for the transition metal TM is predominant in both of the layers (hereinafter the state in which the sublattice magnetization Ms.sub.TM for the transition metal TM is predominant is referred to as TM rich).
On the other hand, for the dual layer film medium of the A type, as shown in FIGS. 3(a) and (b), one of the layers is RE rich and the other of the layers is TM rich in the two layers of the reading layer RL and the writing layer WL. For the sake of the simplicity, explanation will be made hereinafter assuming that both of the reading layer RL and the writing layer WL are TM rich for the P type and that the reading layer RL is TM rich, while the writing layer WL is RE rich for the A type.
In the case of conducting recording to the dual layer film medium of the P type, the directions for the respective magnetization Ms.sub.1 and Ms.sub.2 in the reading layer RL and the writing layer WL of the medium are arranged previously in one identical direction as shown in FIG. 4(a) and then an external auxiliary magnetic field Hex is applied downwardly in the figure while heating a recording area shown by hatched lines by means of a laser beam, by which the magnetization Ms.sub.2 and Ms.sub.1 in the writing layer WL and the reading layer RL are turned downwardly in the figure. Then, by way of the temperature lowering process, downward magnetization Ms.sub.2 and Ms.sub.1 appear in the writing layer Wl and the reading layer RL.
On the other hand, in the case of conducting recording to the dual film medium of the A type, magnetization Ms.sub.1 in the reading layer RL and the magnetization Ms.sub.2 in the writing layer WL of the medium are arranged, respectively, each in one direction and such that the directions are opposite to each other between the reading layer RL and the writing layer WL. Then, when an external auxiliary magnetic field Hex is applied downwardly in the figure while heating the hatched area by means of the laser beam as shown in FIG. 5(b), the magnetization Ms.sub.2 in the writing layer WL is reversed downwardly, whereas the magnetization Ms.sub.1 in the reading layer RL remains as it is in the downward direction. Then, in the temperature lowering process after completion of the laser beam irradiation, magnetization Ms.sub.1 in the reading layer RL turned upwardly in the figure due to the exchange coupling between the two layers and information is recorded on the hatched portion in the reading layer RL.
By the way, in order to obtain excellent recording characteristics in the opto-magnetic recording medium, it is necessary that the shape of bits written into the reading layer RL has a regular and well-arranged shape. For this purpose, it is necessary in the case of the dual layer film medium of the P type that bits, when recorded in the reading layer RL, do not diffuse into other region than the area of the writing layer WL that reaches the temperature near the curie point (laser beam irradiated area) by the exchanging force between the two layers.
In the case of the dual layer film medium of the A type, it is necessary to form regular and well-arranged bits in the writing layer WL and transfer them to the reading layer RL.
Then, necessary conditions required for each of the P and A types in each of the recording and temperature lowering processes are as shown in Table 1 according to the theoretical calculations.
TABLE 1 ______________________________________ Recording Type Hex = H.sub.B ______________________________________ ##STR1## A ##STR2## ______________________________________ Temperature lowering Type Hex = 0 Hex = H.sub.B ______________________________________ P Ms.sub.2 h.sub.2 H.sub.2 &gt; Ms.sub.1 h.sub.1 H.sub.1 ##STR3## A Ms.sub.2 h.sub. 2 H.sub.2 &gt; Ms.sub.1 h.sub.1 H.sub.1 ##STR4## ______________________________________
From Table 1, it is necessary as a measure for the development of the dual layer film medium, to make the magnetization Ms.sub.1 in the reading layer RL smaller, the magnetization Ms.sub.2 in the writing layer WL larger and, further, to establish a relationship between Ms.sub.1 and Ms.sub.2 including H.sub.1, H.sub.2, h.sub.1, h.sub.2 as: EQU Ms.sub.2 .times.H.sub.2 .times.h.sub.2 &gt;Ms.sub.1 .times.H.sub.1 .times.h.sub.1
H.sub.1. . . coersive force of the reading layer RL at that temperature. PA1 H.sub.2 . . . coersive force of the writing layer WL at that temperature. PA1 h.sub.1 . . . film thickness of the reading layer RL PA1 h.sub.2 . . . film thickness of the writing layer WL PA1 .sigma.w . . . boundary magnetic wall energy at a temperature in this case PA1 H.sub.ex . . . external auxiliary magnetic field (applied in the direction opposite to that for the magnetization in the writing layer WL)
By the way, the film thickness h.sub.2 of the writing layer WL has to be within such a range as capable of undergoing the exchanging force from the boundary between both of the layers and, if the magnetization Ms.sub.2 in the writing layer WL is excessively large, a stray magnetic field or demagnetizing field is increased to cause distortion in the shape of the recorded bits even if in the curie point recording.
On the other hand, the reading layer RL is required to have a great Kerr rotation angle .theta.k, and small saturation magnetization Ms and coersive force Hc. On the other hand, GdFeCo among the reading layer media used at present has a rather great Kerr rotational angle .theta.k of about 0.46.degree. (at a measuring wavelength of 780 nm), in which the saturation magnetization Ms and the coersive force Hc are about 50 emu/cm.sup.3 and 0.5 KOe, respectively. Accordingly, for improving the C/N ratio further, it is necessary to develop a medium having a greater Kerr rotational angle .theta.k than that of GdFeCo while suppressing Ms and Hc.
The present inventor has sought for media adaptible to the conditions as described above among quaternary rare earth-transition metal opto-magnetic recording media and has found that GdDyFeCo is particularly excellent as the reading layer medium.
More specifically, in the following formula: (Gd.sub.100-X Dy.sub.X).sub.Z (Fe.sub.100-Y Co.sub.Y).sub.100-Z, the Kerr rotation angle .theta.k is increased to greater than 0.48.degree. within a range: X, Y, Z as X=5-25 atm %, Y=18-30 atm % and Z=16-24 atm %. In particular, the Kerr rotational angle .theta.k can be increased to 0.54 while suppressing the saturation magnetization Ms to less than 250 emu/cm.sup.3 in a TM rich composition ratio in which X is about 20 atm % and the coersive force Hc is less than 2 KOe. The Kerr rotational angle .theta.k is increased by 17.4% as compared with that of usual CdFeCo.
By the way, although the value for the saturation magnetization Ms can be suppressed relatively low as 250 emu/cm.sup.3, it is rather increased as compared with that of the conventional reading layer medium. Therefore, in order to form regular and well-arranged bits while satisfying the conditions in Table 1, it is necessary that the saturation magnetization Ms in the writing layer WL is also made greater relatively.
Further, a large coersive force Hc is required for the writing layer medium and, at the same time, a lower curie temperature Tc is required for the improvement of the recording sensitivity. Accordingly, a medium having a curie temperature Tc between 100 .degree. to 150.degree. C. has predominantly been used. However, according to the study of the present inventor, it has been found that there is no practical problems in view of the recording sensitivity so long as the curie temperature Tc is up to about 190.degree. C. Accordingly, the range for selecting the writing layer can be widened as compared with the usual case.
Then, as a result of our search under the foregoing conditions, the inventors have been found that TbFeCo is excellent as the writing layer. In particular, in the following formula representing the composition ratio: EQU Tb.sub.X (Fe.sub.100-Y Co.sub.Y).sub.100-X
both of the saturation magnetization Ms and the coersive force Hc could be increased while restricting the curie temperature Tc to 155.degree.-195.degree. C. by defining X as 15 to 30 atom % and Y as 6 to 12 atom %, respectively.
In view of the relationship with the recording process described later, if the saturation magnetization Ms for the reading layer medium is large, it is necessary that TbFeCo has RE rich composition ratio near the compensation composition with X being greater than 25 atom %, and such a composition ratio that predominant magnetization does not turn from RE into TM in the temperature elevation process due to the irradiation of laser beams upon recording. If a portion of Tb in TbFeCo is substituted with other rare earth element such as Ge, Dy, Ho or Nd, for controlling the curie temperature, TbFeCo can be used sufficiently as the writing layer medium, so long as RE rich composition ratio is kept.
Then, for suppressing writing noises, the inventors have examined as to which of the P or A type film medium is suitable.
In the dual layer film medium of the P type in which the directions of magnetization Ms.sub.1, Ms.sub.2 in the reading layer RL and the writing layer WL are in parallel with each other, bits formed by the external auxiliary magnetic field and the stray magnetic field to the laser beam irradiated area of the reading layer RL upon recording undergo the effect of exchanging force between both of the layers and, accordingly, are not diffused to the outside of the area of the writing layer WL that reaches a temperature near the curie point (laser beam irradiation area), but can take any optional shape within a range near the curie temperature.
Accordingly, if the saturation magnetization Ms of the reading layer RL is large, the bits formed in the reading layer RL are disturbed at the outer periphery or tend to form multiple polymagnetic domains so as to reduce the magnetostatic energy within the region near the curie temperature of the writing layer WL. The bits thus formed irregularly also form irregular shape of bits in the writing layer WL in the temperature lowering process. Accordingly, in the case of using a medium of large saturated magnetization Ms as in the present invention, the P type dual layer film medium is not suitable.
Then, in the case of using the A type dual layer film medium in which the directions of the magnetization Ms.sub.1, Ms.sub.2 of the reading layer RL and the writing layer WL are opposite to each other, since the magnetization Ms.sub.1 of the reading layer RL has the same direction with that of the external auxiliary magnetic field Hex upon forming bits in the writing layer WL during recording, as shown in FIG. 5(b), bits are not formed in the reading layer RL during irradiation of the laser beam. Then, the bits can be formed in the reading layer RL only by means of the transfer of regular and well-arranged bits having been formed in the writing layer WL by the curie point recording, in the course of the temperature lowering process to the reading layer RL by the exchanging force. Thus, the conditions shown in Table 1 should be satisfied so as to realize the transfer for the A type.
As has been described above, improvement for the exchange-coupling force between the two layers is necessary for reducing the writing noises in the dual layer film structure as has been pointed out, but it is not yet still sufficient. The A type dual layer film structure is considered to be preferably for a medium having a large saturation magnetization with an aim of improving the Kerr rotation angle .theta.k for the reading layer RL.